1. Field of the Invention
This invention relates to a solar heat collector for a hot water supply, solar heat air conditioning systems and the like.
2. Description of the Prior Art
Heretofore, there have been proposed various types of solar heat collectors for the hot water supply, solar heat air conditioning systems and the like.
FIG. 1 is an explanatory view illustrating a solar heat collector 1 of the prior art. This solar heat collector 1 comprises: an absorber plate 2 for converting the solar rays into thermal energy to transmit the thermal energy to a heat transfer medium such as water; a heat transfer medium flow path 3 formed in the absorber plate 2; a transparent sheet 4 made of glass or the like for preventing the convection heat loss through the absorber plate 2 and protecting the absorber plate 2 against the contaminatipn and damages caused by the external factors; a heat insulating material 5 for preventing the heat loss through the rear surface of the solar heat collector; and an outer box 6 for protecting the abosrber plate 2 and the insulating material 5, totally covering the solar heat collector in cooperation with the transparent sheet 4.
The performance (i.e. efficiency) of the abovedescribed solar heat collector is determined by the extent of suppressing the heat loss: the heat loss would be caused in a manner that a part of the abosorbed energy which has been converted to the thermal energy from the solar rays at the absorber plate 2 of the solar heat collector is discharged through the transparent sheet 4 to the outside of the solar heat collector by the mechanisms of convection, radiation and conduction. As the methods of suppressing the heat loss described above, there have been proposed the following ones:
(1) The surface of the absorber plate 2 is subjected to a selective absorption surface treatment for suppressing the radiation heat loss. This treatment prevents the infrared rays from radiating from the absorber plate 2 toward the transparent sheet 4; the radition of the infrared rays is caused by the thermal energy to which the solar rays are converted at the absorber plate 2. Thus, this treatment suppresses the radiation heat loss from the absorber plate 2 toward the transparent sheet 4.
(2) A convection preventive structure such as a honeycomb transparent heat trap for suppressing the convection heat loss is provided between the absorber plate 2 and the transparent sheet 4. The convection heat loss is caused by the following mechanism: there occurs the temperature difference between the high-temperature absorber plate 2 and the low-temperature transparent sheet 4; a convection current is generated in the space between the absorber plate 2 and the transparent sheet 4; and the thermal energy of the absorber plate 2 is transferred to the transparent sheet 4 to cause heat loss. Such convection heat loss is suppressed by the provision of the convection preventive structure in the space between the absorber plate 2 and the transparent sheet 4. This method is disclosed in U.S. Pat. No. 4,019,496.
(3) There is utilized a vacuum heat insulating system for controlling the conductio heat loss and the convection heat loss.
Now, the abovedescribed conventional methods of suppressing the heat loss in the solar heat collectors have displayed effects in suppressing the heat losses to some extent, however, the three forms of heat losses due to the convection, radiation and conduction have such causal relations that, when one of these three losses is suppressed, the heat losses due to the remaining causes are accordingly increased, and, when two of these three heat losses are suppressed, the heat loss due to the remaining cause is accordingly increased.
For example, in the solar heat collector 1 shown in FIG. 1, when the surface of the absorber plate 2 is subjected to the selective absorption surface, the quantity of radiation heat from the absorber plate 2 to the transparent sheet 4 is decreased, however, with the result that the transparent sheet 4 is lowered in temperature, thus resulting in increased quantity of convectio heat from the absorber plate 2 to the transparent sheet 4. As the measures of preventing such increased quantity of convectio heat from the absorber plate to the transparent sheet, there has been proposed that a transparent (for the solar rays) heat trap having a thickness of 20 to 100 .mu.m is provided between the transparent sheet and the absorber plate subjected to the selective absoption surface treatment, as disclosed in U.S. Pat. No. 4,294,198. It is admitted that the convection between the abosorber plate and the transparent sheet is decreased with this measures. This measures, however, encounters the following problems: the above heat trap is transparent for the solar rays but not transparent for the infrared rays, i.e. a small extent of the infrared radiation is not expected; and thus the thermal energy transferred from the absorber plate to the heat trap by the convection will now radiates as the infrared rays from the heat trap toward the transparent sheet.
On the other hand, in the cae of the conventional vacuum cylindrical solar heat collector 1A shown in FIGS. 2(A) and 2(B), when the surface of the absorber plate 2 is formed into the selective absorptio surface and a gap 9 for heat insulating disposed between the absorber plate 7 and a transparent cover 8 is vacuum-insulated, the heat losses in two forms of the radiation and convection can be suppressed, however, the heat loss through a vacuum sealed potion 10 due to the conduction is increased to a great extent.
Consequently, as described in the two examples shown above, the conventional methods of suppressing the heat loss in the solar heat collectors display the effects to some extent, however, present such a disadvantage that the heat losses of the three forms cannot be suppressed at the same time.